HR: 1340h
AN: T53A-1408    [Abstracts]
TI: Crustal Growth of the Izu - Ogasawara (Bonin) - Mariana Oceanic Island arc Inferred from Seismic Velocity Structures
AU: * Takahashi, N
EM: narumi@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showwa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kaiho, Y
EM: kaihoy@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showwa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Sato, T
EM: tsato@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showwa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Fujie, G
EM: fujie@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showwa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kodaira, S
EM: kodaira@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showwa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showwa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan
AB: It is said that ancient earth consists of the basaltic materials without granitic ones, despite of current large continents has thick granitic materials. Because the oceanic island arc has andesitic middle crust despite of the initial arc has no andesitic materials, the arc is regarded as one of steps of crustal evolution to a continent. Suyehiro et al. (1996), Takahashi et al. (2004) and Crawford et al. (2003) found thick andesitic middle crust with velocity of 6 km/s in northern Izu, Mariana, and Tonga arcs, respectively. However, the central and the eastern Aleutian arcs have little or no andesitic middle crust (Holbrook et al., 1999). To understand the nature of the crustal growth, we have to clarify not only the andesitic middle crust but also the lower crust and the upper mantle, because repeated crustal differentiation may occur within the middle and lower crusts. In 2005, JAMSTEC performed an active seismic experiment across the southern Ogasawara (Bonin) arc using an airgun array and 110 ocean bottom seismographs. A seismic main line runs perpendicularly across the Eocene arc of initial oceanic island arc (the Ogasawara ridge), the current active arc with non-mature and the old Miocene arc. From the preliminary results, the three arcs have similar crustal thickness. However, these crustal components seem to be different each other. The Eocene arc has relative thin middle crust and thick lower crust. The current active arc has thin middle crust and thick lower crust with high velocity of over 7 km/s. The Miocene arc has thick middle crust similar to the northern Izu arc. The Ogasawara trough, which is regarded as a part of Oligocene arc just before the backarc opening, has a crustal thickness of 15-20 km. Moreover, according to reflection imaging recorded by OBSs, we clarified the large fault between the current active arc and the Ogasawara trough, which cuts entire of the crust. This fault suggests that a process of the initial stage of the backarc opening might follow the simple shear mechanism. Using above three velocity models within this arc, we discuss crustal transformation accompanied to the crustal growth to the upper mantle.
DE: 0902 Computational methods: seismic
DE: 1744 Tectonophysics
DE: 3025 Marine seismics (0935, 7294)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: Fall Meeting 2005